Lead Acid Battery Recovery Part 3

Since the last post (Lead Acid Battery Recovery Part 2), I have run 7 pulsed battery discharge cycles with the following parameters:

• 1.4 A peak current
• 118 KHz Pulse Frequency
• 25% Duty Cycle

The battery capacity did not seem to increase over these cycles. I did over discharge the batteries in several of the cycles (battery voltage dropped below 11.8 V). The battery desulfator concept does not appear to be helping improve the capacity of this recovered battery.

Here is a table summarizing the estimated battery capacity for each of the discharge cycles since I recovered the battery (estimated based on what the capacity would be as interpolated at a battery voltage of 11.8 V).

From the overall data I can conclude a few things:

• At low loads, the battery capacity of the recovered battery is around 40-50%
• At higher loads, the battery capacity of the recovered battery is around 30%
• Some of the data points are outliers. For the 1.4A pulsed loads, the two outliers had their last two data points that are used to estimate the point at which the battery crossed 1.8V significantly far apart in time and well into the overdischarge (beyond 11.8V), so I think there just isn’t a great way to estimate the capacity in these cases (discharge 5 and 7)
• There doesn’t seem to be any increase in capacity from battery desfulfation with the pulsed load

Here is the discharge curve for discharge 11:

The circuit keeps the battery current pretty stable until the battery is beyond the discharge point. The battery voltage fluctuates between 12.5 and 13 V during most of the discharge, and then at the ends starts to drop off. I would expect that the drop off is probably exponential, but to capture the whole curve would require significantly over-discharging the battery so I didn’t do that. It’s reasonably linear in the range from the normal battery voltage until the 11.8 point where I try to stop the discharge cycle.

I measured across the load resistor in the discharge circuit to capture what the battery discharge current pulses look like. The following is a scope capture:

I’m not sure why the dip occurs in the pulse, and why the negative spike occurs when the transistor switches the current off (is current really flowing back in to the battery?). I may experiment a little more to understand this and report in a future post.

That concludes the lead acid battery recovery experiment for now. I learned a lot about lead acid batteries. It is clear that leaving a lead acid battery on the shelf unused for 20 years without maintenance charges will lead to a significant loss in battery capacity, but the battery is still usable. Hopefully I’ll find some experimental uses for this battery and it doesn’t have to go another 20 years!

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